You didn't indicate a specific home, but I think I can generally help. There are many homes in flood plains that rarely, if ever see actual floods. Others may see flooding on a more regular basis. I've seen homes in 100 year flood plains and 500 year flood plains. In theory a home in a 100 year flood plain would only see flooding once every 100 years. In reality, some may see water more often than that.
If you buy a home in a flood plain, you must purchase flood insurance. All loans require a flood certification. Flood insurance comes from the federal government and depending on the location of the home may not be too expensive. I've attached a link to a site where you can learn much more.